Ceramic composites comprised of silicon carbide whiskers and alumina powder are well known to the prior art. Thus, by way of illustration, U.S. Pat. No. 4,543,345 discloses a silicon carbide whisker-alumina ceramic composite with good fracture toughness. U.S. Pat. No. 4,652,413 discloses that silicon carbide whisker-reinfiroced alumina ceramic articles are useful as structural materials for the fabrication of turbocharger rotors, cylinders, bearings, and other components of heat engines.
One problem with the ceramic composites presently available is that they cost from about eighty to about one hundred and twenty dollars per pound, a price which makes them too expensive for most applications.
One of the reasons said composites are so expensive is that silicon carbide whiskers require a substantial amount of energy to produce. Thus, as is disclosed in U.S. Pat. No. 3,375,073 of McMullen, in the manufacture of silicon carbide a mixture of sand and coke is usually reacted in an Acheson-type electric resistance furnace at a temperature of about 2000-2600 degrees centigrade for from about 80 to about 120 hours; thereafter, the silicon carbide must be cooled for about 90 hours.
An exothermic reaction for the production of silicon carbide is desirable. However, as is known to those skilled in the art, such exothermic reactions are difficult to obtain and control.
Thus, for example, U.S. Pat. No. 2,886,454 of Todd discloses that ". . . certain exceedingly troublesome problems and limitations are encountered in any attempt to produce metal carbides of acceptable properties by exothermic procedures." (column 1, lines 68-71). At column 2 of his patent, Todd discloses that "For such an exothermic reaction to be successful, it must produce a sufficiently high temperature to result in the formation of a carbide mass, it being necessary that the temperature exceed the melting point of the carbide product by several hundred degrees. It has been found that most mixtures of metal oxides, aluminum and carbon that would occur to a person skilled in the art fail completely to produce a carbide mass, and more often than not, when such a mixture is repeatedly varied until finally a mass is produced, the mass contains too much aluminum in addition to the metal carbides or for some other reason lacks the properties required in a product for the present purposes. One difficulty in attempting to theorize as to whether a particular mixture will be operative is that the product is a complex of compounds that do not reveal ordinary valence relationships. Another difficulty is that the attainable temperature is reduced by heat losses arising from such causes as the escape of carbon monoxide and the evaporation of ingredients in the mixture. These losses are difficult to estimate. A further complication is that some of the carbon, as well as the aluminum, reduces the metal oxide, and the reducing action by the carbon lowers the attainable temperature. In the first place, the reduction of an oxide by the carbon results in the generation of less heat than can be produced by the reduction of the oxide by aluminum; and, in the second place, there is substantial heat loss involved in the escape of the resulting carbon monoxide . . . "
One possible exothermic method for the production of silicon carbide was disclosed in applicant George Hida's thesis. Hida's thesis discussed the possibility of producing silicon carbide dendrites together with alumina powder in a single step reaction. In this thesis, which was entitled "Study of Solid-State Aluminothermal Reactions: Influence of Activation and Moderation Processes," which was submitted to the Senate of the Technicron--Israel Institute of Technology in Haifa, Israel in February of 1987, and which was published in April of 1987, a reaction was disclosed in which three moles of silica were reacted with four moles of aluminum and three moles of carbon. The silica reagent used was 200/230 mesh fraction quartz sand; the aluminum reagent used was -325 mesh aluminum powder; and the carbon reagent used was carbon black. Stoichiometric amounts of these reagents, based upon the use of three moles of silica, four moles of alumina, and three moles of carbon, were mixed; the mixture was compacted by cold pressing it in a die; and the pressed samples were placed in a furnace which had been preheated to 720 degrees and were thereafter ignited.
Hida's thesis cited references indicating that, in his process, the presence of carbon reduced the exothermicity. See, e.g., Walton, J. D. and Poulos, N. E., "Cermets from Thermite Reactions," Journal of the American Ceramic Society, Vol. 42, No. 1, pp. 40-49, January 1959. Also see Cutler, R. A., "Synthesis of Submicron Silicon Carbide," presented at the DARPA/ARMY Symposium on Self-Propagating High Temperature Synthesis, Daytona Beach, Fla., U.S.A., October, 1985. Also see references numbers 37, 38, 48, 57, and 67 cited in said thesis.
When the process and stoichiometry suggested in Hida's 1987 thesis is used to prepare silicon carbide whiskers, a relatively low yield (usually about 10 percent) of such whiskers is obtained.
It is an object of this invention to provide an improved exothermic process for preparing a silicon carbide whisker/alumina composite precursor in high yield at a cost which is substantially cheaper than is currently possible.